(1) Field of the Invention
The present invention relates to a medical bag. In particular, it relates to a medical bag capable of receiving an infusion fluid, an elemental diet (hereinafter referred to simply as ED), an antibiotic or the like used in a closed medical system, and a method for preparing the medical bag.
More particularly, the present invention relates to a medical bag in which a mouth portion and a receiving portion are bonded together by high frequency (hereinafter may also be denoted HF) induction heating, and a method for preparing the medical bag.
(2) Description of the Prior Art
In recent years, the employment of a closed system increases in infusion therapy and the like in order to prevent liquid contents therein from contacting the outside. A medical bag is used in this closed system, and the liquid contents in the medical bag are discharged therefrom under the influence of gravity on the contents themselves as well as the structure of the bag and the flexibility of a material of the bag. Further, this kind of medical bag must have heat resistance which can withstand autoclave sterilization to sterilize the contents therein. It is preferred that the medical bag be made from a transparent material so that the contents therein can be monitored from the,outside. For this purpose, flexible polyvinyl chloride resin and ethylene-vinyl acetate copolymer are mainly used as raw materials for the medical bags.
In addition, there is a modified medical bag in which the mouth member and the receiving member are made from a polyolefin resin and a bond portion of the mouth member is formed into a boat shape or the like so as to enable heat sealing.
On the other hand, ED and antibiotics are hermetically preserved in the form of a freeze-dried powder in aluminum packs, glass bottles and the like and are prepared when used, and therefore they could not be utilized in the closed system.
With regard to a bonding process which uses radiofrequency induction heating, when materials to be bonded are ferromagnetic substances, there is generally used a process which comprises first interposing a thermoplastic resin adhesive between these materials, and then causing the ferromagnetic materials to generate heat in a magnetic field, whereby the thermoplastic resin adhesive is melted and the materials are thus bonded to each other; and when materials to be bonded are non-ferromagnetic substances, there is generally used a process which comprises first interposing a thermoplastic resin adhesive containing a magnetic substance between these materials, and then melting the adhesive in a magnetic field, whereby the materials are bonded to each other.
The conventional medical bags are often made from flexible polyvinyl chloride and ethylene-vinyl acetate copolymer. This reason is that flexible polyvinyl chloride and ethylene-vinyl acetate have a high dielectric loss and thus the mouth portion and the receiving portion of the medical bag can be easily bonded together by means of the radiofrequency induction heating. However, flexible polyvinyl chloride contains a great deal of a plasticizer, and therefore there is the problem of safety that the plasticizer might dissolve into the liquid contents in the medical bag. As for ethylene-vinyl acetate copolymer, acetic acid might separate out therefrom at the time of molding and similarly might dissolve into the liquid contents therein, and furthermore, ethylene-vinyl acetate copolymer, if not treated additionally, does not have such heat resistance as to withstand autoclave sterilization. Hence, it is necessary that ethylene-vinyl acetate copolymer be irradiated with a radiation in order to form a crosslinked structure therein, thereby providing the copolymer with the heat resistance. In the conventional manufacturing method of the bags, such an additional process has been required. Now, safe and inexpensive polyolefin resins, having excellent physical properties, are demanded.
However, it has been extremely difficult to manufacture desired medical bags by the use of such polyolefin resins. In general, the polyolefin resin does not release heat enough to melt itself even when a high-frequency electric field is applied, because the dielectric loss of the polyolefin resin is low. In consequence, the polyolefin resin cannot be sealed by high-frequency bonding, in constrast to flexible polyvinyl chloride, ethylene-acetic acid copolymer and the like. For example, when two laminated polyolefin sheets of the medical bag are melted and bonded to a mouth member having a cylindrical outer periphery or the like by means of impulse sealing or heat sealing, it is difficult to melt and bond the sheet laminate and a material of the mouth member which are different in wall thickness, since the impulse sealing, which is in contrast to the high-frequency sealing, is not effective to melt and bond the materials having a complex shape, and since in the heat sealing, heat is given from the outside. In consequence, the mouth member having a small diameter cannot be joined well to the upper portion of the medical bag. There has been suggested a method in which the bond portion of the mouth member is deformed so that the shape of the bond portion may be as close as possible to a plane, with the intention of facilitating the bond of the mouth member and the receiving portion. In this case, however, the seal of the bonded portion is incomplete and the liquid contents therein leak out on occasion, and in addition there is the problem that the manufacturing cost is high.